The
Manhattan GMAT (MG) Strategy Guides are great. I took the 9 week class through MG and have been using the strategy guides to prepare for the actual test. I have a
Kaplan book as well has a Princeton Review book. The MG books are by far the best.
Data Sufficiency is discussed a little in each mathematics strategy guide. MG provides both a strategy for attacking these problems, as well as the fundamentals to solve them.